In the related art, objective optical systems for performing stereoscopic viewing by forming two optical images with parallax on a single image-acquisition element have been proposed.
As an example of such an objective optical system, Patent Literature 1 discloses an objective optical system provided with, sequentially from the object side, a front-group optical system shared between the left and right (in two directions of parallax) and a pair of, that is, left and right, imaging optical systems.